Formation evaluation logs contain data related to one or more properties of a formation as a function of depth. Many types of formation evaluation logs, e.g., resistivity, acoustic, and nuclear, are recorded by appropriate downhole instruments placed in a housing called a sonde. A logging tool including a sonde and associated electronics to operate the instruments in the sonde is lowered into a wellbore penetrating the formation to measure properties of the formation. To reduce logging time, it is common to include a combination of logging devices in a single logging run. Formation evaluation logs can be recorded while drilling or after drilling a section of the wellbore. Formation evaluation logs can be obtained from an open hole (i.e., an uncased portion of the wellbore) or from a cased hole (i.e., a portion of the wellbore that has had metal casing placed and cemented to protect the open hole from fluids, pressure, wellbore stability problems, or a combination thereof). Formation evaluation logs obtained from cased holes are generally less accurate than formation evaluation logs obtained from open holes but they may be sufficient in some applications, such as in fields where the reservoir is well known.
Wellbores are conventionally drilled using a drillstring. The drillstring generally includes a series of drillpipe and a bottomhole assembly (BHA). The BHA includes at least a drill bit and may further include components that would turn the drill bit at the bottom of the wellbore. Oftentimes, the BHA includes a bit sub, a mud motor, and drill collars. The BHA may also include measurement-while-drilling (MWD)/logging-while-drilling (LWD) tools and other specialized equipment that would enable directional drilling. In conventional drilling, casings are typically installed in the wellbore to prevent the wellbore from caving in or to prevent fluid and pressure from invading the wellbore. The first casing installed is known as the surface casing. This surface casing is followed by one or more intermediate casings and finally by production casing. The diameter of each successive casing installed into the wellbore is smaller than the diameter of the previous casing installed into the wellbore. The drillstring is lowered into the wellbore to drill a new section of the wellbore and then tripped out of the wellbore to allow the casing to be installed in the wellbore. As discussed in further detail below, logging may be conducted in the wellbore while the new section is being drilled or after the new section is drilled or while casing is run to the new section.
Traditionally, open hole formation evaluation logs have been obtained using wireline logging. In wireline logging, the formation properties are measured after a section of a wellbore is drilled but before a casing is run to that section of the wellbore. The operation involves lowering a logging tool to total depth of the wellbore using a wireline (armored electrical cable) wound on a winch drum and then pulling the logging tool out of the wellbore. The logging tool measures formation properties as it is pulled out of the wellbore. As a fallback in hostile environments, the logging tool may also measure formation properties as it is lowered into the wellbore. The wireline transmits the acquired data to the surface. The length of the wireline in the wellbore provides a direct measure of the depth of the logging tool in the wellbore. Wireline logging can provide high quality, high density data quickly and efficiently, but there are situations where wireline logging may be difficult or impossible to run. For example, in highly deviated or horizontal wellbores, gravity is frequently insufficient to allow lowering of the logging tool to total depth by simply unwinding the wireline from the winch drum. In this case, it is necessary to push the logging tool along the well using, for example, a drillpipe, coiled tubing, or the like. This process is difficult, time consuming, and expensive. Another situation where wireline logging may be difficult and risky is in a wellbore with stability problems. In this case, it is usually desirable to immediately run casing to protect the open hole.
LWD is a newer technique than wireline logging. It is used to measure formation properties during drilling of a section of a wellbore, or shortly thereafter. An LWD tool includes logging devices installed in drill collars. The drill collars are integrated into the BHA of the drillstring. During drilling using the drillstring, the logging devices make the formation measurements. The LWD tool records the acquired data in its memory. The recorded data is retrieved when drilling stops and the drillstring is tripped to the surface. During drilling, a subset of the acquired data may be sent to the surface using conventional telemetry systems. LWD data transmitted to the surface in real time may assist in making quick and accurate decisions with respect to directional drilling and hazards prevention. The range of LWD services available and logging speed are limited in comparison to wireline logging. In LWD, logging speed can be limited by the real time data-rate of the MWD tool's telemetry. In this case, the drilling rate may be slowed so that sufficient data can be sent uphole for drilling or formation evaluation decisions. However, LWD has an advantage over wireline logging in that properties of the formation are measured before drilling mud invades the formation deeply. Further, LWD can be used in wellbores that may prove difficult or even impossible to measure with conventional wireline logging. For example, because the LWD tool is part of the drillstring, it can easily log highly deviated and horizontal wellbores, whereas wireline logging may require pushing of the logging tools using drillpipe, coiled tubing, or the like.
Through-bore-logging (TBL) is a much newer technique than LWD. It allows open hole formation evaluation logs to be obtained without tripping the drillstring out of the wellbore. (See, for example, John Runia et al., “Through Bore Drilling Systems: a New Drilling Option,” SPE 79794, February 2003). A typical TBL system includes a drilling string having a drill bit with a removable and re-insertable bit insert and a latch attached to the bit insert. During drilling, the latch is locked into the bit shank. The TBL system further includes a string of logging tools (e.g., gamma, resistivity, density, neutron, and sonic logging tools) and may include a MWD tool to allow real-time data transmission. When the drill bit reaches total depth, the drill string is pulled back and the string of logging tools is run on a slickline or pumped down the bore of the drill string. A special running tool attached to the bottom of the logging tools releases the latch from the bit shank, allowing the bit insert to be released from the drill bit, allowing the logging tools to pass through the drill bit. With the logging tools below the drill bit, logging occurs as the drill string is pulled back from the wellbore. After logging the open hole, the logging tools are pulled through the drill bit with a slickline. The latch locks itself to the bit shank and releases the special running tool at the bottom of the logging tools, allowing the logging tools to be removed from the drill string and drilling to continue.
U.S. Pat. No. 6,119,777 (Runia) describes a method of logging a conventionally drilled wellbore while running a casing into the wellbore. The lower end part of the casing run into the wellbore, referred to as the casing shoe track, is provided with a logging tool. The logging tool is releasably retained in a glass fiber reinforced epoxy (FRE) tube attached to the inner surface of the casing shoe track. In one example, the logging tool is composed of a gamma ray logging device, a neutron logging device, a density logging device, and a power/memory cartridge. Density measurements are made through a window in the casing shoe track formed of FRE. Some sections of the casing shoe track are made of glass FRE to optimize log response of tools affected by steel. In general, the casing shoe track is made of drillable materials so that it can be drilled out if necessary. The casing shoe track also allows through pumping of mud. Logging is conducted as the casing is run into the wellbore. After the casing is installed and prior to cementing the casing in place, a latching device is connected to the logging tool. The latching device is also connected to a wireline or coiled tubing provided with electrical conducting means, thereby allowing acquired data to be transferred from the logging tool to the surface. After transferring the data, the logging tool is retrieved, and the casing is then cemented in place.
A need remains for techniques to obtain open hole formation evaluation logs, particularly where LWD would not be cost-effective and wireline logging could be difficult and/or risky.